Electronic appliances come with a wide variety of functionalities. In many cases it is not advisable to try implementing all possible functionalities within a single device, because the needs and expectations of different users will vary. Rather, it may be more advantageous to provide a host apparatus with a certain set of basic functionalities, and to separately provide a selection of add-on modules that users may acquire according to their individual needs and use together with the host device. In some cases the hardware components needed for implementing a functionality wear out, discharge, fill up, become obsolete, or otherwise achieve a state of reduced usability faster than some other hardware in the apparatus. Implementing such components in an add-on module facilitates changing and replacing them easily without needing to acquire a new complete apparatus. Also other cases exist in which it is advantageous to couple two parts together with an electromechanical connection to form a combined entity. An electromechanical connection is a way of putting together two or more physical entities, implementing both an electric connection for transferring electric signals in at least one direction between at least two of said physical entities and a mechanical connection for mechanically maintaining at least two of said physical entities within an at least temporarily fixed coupling to each other.
As an example we may consider a portable communications device. Different users have widely differing needs for their portable communications devices, which makes it advantageous to provide at least some of the more advanced or more exotic functionalities in separate add-on modules that can be removably attached to a body part of the device. Even a single user may have different needs at different times, which leads to the same conclusion about add-on modules. Also other portable electronic appliances, such as personal digital assistants, palmtop and laptop computers, digital cameras and camcorders, remote controls, and the like may include modular structures. In some cases it is not reasonable to designate one part of the system a body and the other as an add-on module, because the system merely comprises to apparatuses that can be electromechanically connected together to form a larger entity.
The electromechanical connection between a body part and an add-on module (or between two electronic apparatuses) must meet certain expectations. The connection should be robust and reliable, so that the electric coupling operates undisturbed and the mechanical coupling does not easily come loose by accident. On the other hand the connection must be easy to use, so that a human user perceives making the electromechanical connection and detaching it, when desired, as easy and straightforward. Making a wrong connection (e.g. confusing the polarity of an electric connection, or putting one of the connected parts backwards) should be prevented in a natural way. The electromechanical connection should not place unnecessary restrictions to the freedom of designing other characteristics of the connected apparatuses, such as outer appearance, functional capacity, ergonomics, or interoperability with other devices.